Political party affiliation in the United Kingdom

Membership and supporter status within political parties in the United Kingdom typically contain restrictions including:

Practically, involvement in a political party without membership may limit an individual's involvement. Parties that offer a lower level of affiliation, such as supporter status tend to have looser constraints.

Membership of political parties has been in decline in the UK since the 1950s, falling by over 65% from 1983 (4 per cent of the electorate) to 2005 (1.3 per cent). Typically party affiliation allows an individual to support only a single party. However a notable exception to this is the Co-operative Party who supports only Labour and the SDLP and since 2016, More United which supports liberal and progressive candidates.

Some political parties in the UK disclose their membership, some do not, there is no legal or electoral obligation to give figures let alone accurate figures.

"Historically the Conservative Party has not produced systematic membership estimates, though in recent years Conservative Home has quoted estimates from CCHQ. This reluctance to publish estimates is due to the structure of the party and the process via which individuals become full members".

There are few high-profile cases of membership criteria enforcement and as such tend to happen under exceptional circumstances.

During the 2015 Labour party leadership election it emerged that 260 former candidates from the Green Party, Left Unity and the Trade Union and Socialist Coalition had attempted to become registered supporters but will now be blocked from voting. Shortly before this it was revealed that Conservative MP and former junior minister Tim Loughton had been caught applying to become a registered Labour supporter, subsequently claiming that his intention was to "blow the gaff on what a complete farce the whole thing is". Veteran Labour MP Barry Sheerman also joined calls for the election to be "paused" over the fears of infiltration by other parties. The Labour Party told representatives of the four candidates at a meeting on 11 August that 1,200 members and supporters of other parties had been excluded and a further 800 were under investigation. Harriet Harman at the time admitted that as many as 100,000 people may be blocked from voting.

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